1. Field of the Invention
The present invention relates to an apparatus and method for reception in a multi-input multi-output (MIMO) system; and, more particularly, to an apparatus and method for MIMO scheme using a plurality of transmission/reception (TX/RX) antennas in an orthogonal frequency division multiple access (OFDMA) system.
2. Description of Related Art
Recently, the weigh of data services is increasing in wireless communication systems, together with conventional voice services. In the data services, multimedia data based on a plurality of media are increasing in comparison with data based on a single medium. Also, the wireless communication systems are evolving into a broadband mode to transmit high-rate data. An orthogonal frequency division multiplex (OFDM) scheme is being used as a transmission scheme in a wireless communication system for transmission of high-rate broadband data. Also, a multi-input multi-output (MIMO) scheme, which transmits many data simultaneously by means of a plurality of TX/RX antennas, is being proposed to increase the frequency efficiency.
At present, the OFDM scheme is a modulation scheme that is most suitable for high-rate data transmission. The OFDM scheme can be regarded as a special mode of multi-carrier transmission, and transmits one data stream through a plurality of subcarriers, not through a single carrier.
In wireless communication environments, a delay spread occurs due to a multipath caused by obstacles such as buildings. When a delay spread time is longer than a transmission time of the next symbol, an inter-symbol interference (ISI) occurs. In this case, a frequency selective fading occurs in a frequency domain. In the case of using a single carrier, an equalizer is used to remove an inter-symbol interference. However, the complexity of an equalizer increases as the data rate increases.
In the OFDM scheme, high-rate data are transmitted in parallel by means of a plurality of subcarriers, thereby preventing a frequency selective fading of one carrier in each subcarrier. Also, in order to prevent an orthogonal component from being obscured due to an inter-symbol interference caused by a wave delayed during transmission, a guard interval is inserted to prevent the interference. The OFDM scheme is being used as the core technology for various fields such as 802.11 Wireless LAN, DMB (Digital Multimedia Broadcasting), PLC (Power Line Communication), xDSL, 4G mobile communication, and HPi (High-Speed Portable Internet).
The conventional wireless communication system provides voice-centered services, and uses channel coding to overcome the poorness of a channel. However, as a high-quality multimedia service capable of communication with everybody everywhere is required, the communication services evolve from voice-centered into data-centered. What is therefore required is a technique to transmit a large amount of data rapidly and errorlessly. However, a mobile communication environment may greatly distort signals due to multipath, shadow effects, propagation attenuation, and interference. In particular, fading due to multipath may cause a severe distortion by the sum of signals with different amplitudes and phases received through different paths. This fading is one of the limitations that must be overcome in high-rate digital communication systems. One of the methods proposed to solve the above limitation is an MIMO system.
The MIMO system is a development of a single-input single-output (SISO) system. The MIMO system uses a plurality of TX/RX antennas. Because the MIMO system transmits several signals simultaneously by several antennas, it can transmit more data without increasing the bandwidth, in comparison with the convention system.
However, the MIMO system is weak against frequency selective fading and inter-symbol interference that occur during high-rate transmission. In order to overcome the above limitation, an OFDM scheme is used together with the MIMO system. The OFDM system processes data in parallel, divides a high-rate data stream into low-rate data streams, and transmits the data streams simultaneously by means of a plurality of carriers. The use of low-rate parallel carriers increases a symbol interval, thus reducing the inter-symbol interference. Also, the OFDM system uses a guard interval, thereby removing the inter-symbol interference nearly completely. Also, because the OFDM system uses a plurality of carriers, it is robust against frequency selective fading. Consequently, a combination of the two systems can utilize the advantages of the MIMO system and can overcome the limitations by means of the OFDM system.
However, an OFDM receiver must calculate an LLR value in order to decode a signal. The LLR value calculation requires multipliers, thus increasing the complexity of a decoder. What is therefore required is a method for reducing the complexity of a hardware structure used to calculate an LLR value in a decoding operation of the receiver.